1. Field of the Invention
The invention relates to a method for detecting a direction of rotation and a direction of rotation of a rotor.
2. Description of the Background Art
Method and devices suitable for carrying out the method for counting the rotations of a rotor, for example, of an indicator or a disk in the measuring mechanism of a consumption meter, are known in numerous variations and are commercially available. Thus, for example, DE 33 40 508 C2 discloses a pulse generator device for water meters with standard pointer-roller counting mechanism. A permanent magnet is mounted on a supporting disk as a pulse transmitter. A pulse receiver arranged outside the counting mechanism housing emits a metering pulse with each passage of the permanent magnet. A comparable arrangement is the subject matter of DE 100 60 198 A1. This type of pulse generator device has not proven useful in practice, however, because it cannot be distinguished whether the counting mechanism is running forwards or backwards. Very large errors occur if the pulse transmitter swings in front of the pulse receiver; then metering pulses are constantly generated although the counting mechanism has not completed a revolution forwards or backwards.
The use of inductive proximity sensors is widespread in practice, which use an LC resonant circuit that can be damped by the proximity of an electrically conducting device and an evaluation circuit for determining the vibration damping triggered by the damping element. The LC resonant circuit is thereby periodically excited to resonant vibrations in the rhythm of a scanning frequency. Subsequently, the time is measured until the amplitude of the resonant vibrations falls below a previously established threshold. If the damping element is located in the sensing range of the sensor, the decay time is short, if the damping element is located outside the sensing range, the decay time is long. If the damping element is located partially in the sensing range of the sensor, the decay time is between these two extremes; the decision on whether the damping element is inside or outside the sensing range is achieved by an additionally introduced switching threshold.
Details on this are shown by the following documents: DE 36 11 862 A1, DE 37 33 943 A1, DE 37 33 944 C2, EP 0 608 494 B1, EP 0 467 753 B1 (which corresponds to U.S. Pat. No. 5,187,989), DE 39 23 398 C1 and DE 19 809 031 A1.
A further property of the known methods and devices is the number of sensors used. Thus, for example, DE 37 33 943 A1 and DE 37 33 944 C2 show methods and devices that use only one sensor. Likewise, DE 36 11 862 A1 shows only one sensor, which is embodied in addition as a transformer.
EP 0 608 494 B1 shows a rotation detector that operates with a measurement sensor, a reference sensor and a rotor with three different damping properties.
EP 0 467 753 B1 shows a device for detecting rotation, which operates with two and more measurement sensors, wherein the sensors are energized consecutively. The inductive crosstalk between the sensors, which can cause disturbances, is thereby to be avoided.
DE 39 23 398 C1 discloses a rotation detector for battery operation, which operates with even four sensors, respectively two of which are connected in a complementary manner to one another. Since the excitation of four LC resonant circuits naturally uses more battery current than the excitation of one or two sensors, this circuit can be operated only with a large battery or only for a short time.
DE 198 09 031 A1 shows that it is possible to produce three coils, which respectively are to be supplemented by a capacitor to an LC resonant circuit, as a printed circuit.
The scanning frequency is not least decisive for the consumption of battery current. The higher the scanning frequency, that is, the more often per time unit the resonant circuits are excited, the higher the battery consumption. To reduce the battery current, the scanning frequency should be as low as possible. However, Shannon's Theorem must not be contravened. Shannon's Theorem says that for the reliable detection of the rotor rotation for each revolution more than two scans must be carried out. This means that the scanning frequency has to be coordinated with the highest speed to be expected, although the maximum speed is rarely or never reached.
To reduce the battery consumption in these cases, it is known from EP 0 898 152 A1 to change the scanning frequency adaptively. When the rotor stops, the scanning frequency is reduced in a stepwise manner, as soon as the rotor rotates again, the maximum scanning frequency is reestablished. However, since the electronic system does not recognize the rotor restarting until it has completed at least a quarter of a rotation, faulty measurements can occur, until the full scanning frequency has been reached again.
If two sensors are used in order to detect the revolutions of a disk that is half covered by a damping element, the two sensors are offset to one another by π/2. With the aid of the above-mentioned switching threshold it is detected whether the damping element is located in front of the sensor or not. Each sensor thus emits two signals: damped or undamped. Four states can be determined therefrom, according to the four quadrants of the disk. By comparing the consecutive signal modes it can be detected whether the disk has respectively rotated forwards or backwards by π/2. After counting four consecutive forward movements, a full revolution forwards is detected, after counting four consecutive backward movements, a full revolution backwards is detected.
The important disadvantage of these circuit arrangements is the uncertainty of the decision “damped” or “undamped,” if only the edge of the damping element is located in the region of the sensor sensing range. In addition, there is the general inaccuracy in the detection of the instantaneous decay time, caused by the inaccuracy in the measurement of the oscillation amplitudes and the decision on whether the current amplitude is above or below the amplitude threshold. Further error sources are noise, e.g., the quantization noise, and the changes of the sensors due to aging. All of this means that the scanning frequency in practice has to be set much higher than required by Shannon's Scanning Theorem. This is associated with correspondingly increased battery consumption. This is disadvantageous.